1. Field of the Invention
The invention relates to a receiver for use in a system for transmitting data symbols at a given Baud rate, the receiver comprising a clock oscillator which has a control input for controlling the phase and frequency of a locally generated Baud rate clock signal, and means for deriving an error signal which is a measure of the phase difference between a Baud rate clock signal belonging to the received data symbols and the Baud rate clock signal generated locally by the clock oscillator.
A receiver of this type is known from Dutch Patent Application No. 8800490 which corresponds to U.S. Pat. No. 4,959,845, issued Sep. 25, 1990 laid open to public inspection.
2. Description of the Related Art
In such prior-art receiver the received data signal is sampled by the locally generated Baud rate clock signal, whose frequency is equal to that of the symbol rate of the transmitted data symbols, so as to make decisions on the logic value of the output symbols at the symbol rate.
In order to obtain a correct symbol decision, it is desirable that the magnitude of the phase difference between the Baud rate clock signal belonging to the received data symbols and the locally generated Baud rate clock signal remain below a specific value. This is realised in the prior-art receiver by measuring the phase difference between the clock signals and adapting the phase of the locally generated Baud rate clock signal in a step-by-step fashion on the basis of the result of this measurement, so as to reduce the phase difference between the clock signals. For that purpose, such receiver comprises a phase control circuit.
The prior-art measurement of the phase difference between the clock signals is based on the experience that the shape of the leading edge of a received data symbol, also termed the precursor, is virtually only dependent on the filters in the transmitter and the receiver and virtually independent of the properties of the transmission line in between.
As described in above-mentioned patent, an error signal is obtained which is a measure of the phase difference between the clock signals, by subtracting a given fraction of the present value of the received signal from the value thereof at the previous sampling instant. This is possible because the nominal sampling error signal which is not dependent on the phase difference between the clock signals, but is dependent for example, on echos and/or intersymbol interference, is sufficiently low.
In full-duplex two-wire transmission the received data signal may be disturbed by echos from symbols simultaneously transmitted by the receiving station. A first type in echo may develop as a result of crosstalk of the so-called hybrid circuit, as a result of which although the transmitted symbols which end up in the receiver are weaker, admittedly, they are still stronger than the received signal. A second type of echo may develop because the transmission line at the far end is not terminated with a perfectly matched load, as a result of which the transmitted symbols are partly reflected at the far end of the transmission line and thus return to the receiver through the transmission line. By reproducing these echo signals by means of an adaptive echo canceller on the basis of the transmitted symbols and the error signal as well, and by subtracting the reproduced echos from the received data signal, the disturbance of the received data signal as a result of echos may be reduced considerably.
Another possible source of the disturbance of the received data signal is formed by intersymbol interference, which develops from the transmission properties of the transmission line, as a result of which the values of the signals received at previous sampling instants still affect the value of the received data signal at the present sampling instant. This source of disturbance may be reduced by reproducing the intersymbol interference by means of an adaptive decision feedback intersymbol interference canceller, and subtracting from the received data signal the intersymbol interference thus reproduced.
A relatively small phase step of the locally generated Baud rate clock signal may lead to a misadaptation of the adaptive echo canceller or the adaptive decision feedback intersymbol interference canceller, which may result in a greater probability of incorrect symbol detection. Thus, it is desirable that no phase steps be made after the capturing of the phase control circuit. This is achieved by switching the phase control circuit off when the magnitude of the phase difference between the clock signals becomes smaller than a specific value.
In order to be able to track slow variations in frequency and phase of the received signal, the prior-art clock oscillator is phase-controllable in addition to being frequency-controllable. For this purpose, the error signal is applied to a polarity detector whose output is connected to the control input of the clock oscillator. Consequently, the frequency of the clock oscillator will be increased in one state of the polarity detector and decreased in the other state of the polarity detector.
As long as the locally generated Baud rate clock signal is lagging the Baud rate clock signal associated with the data symbols, the frequency of the prior art local clock oscillator will be increased in small steps. If the locally generated Baud rate clock signal is leading the Baud rate clock signal associated to the data symbols, the frequency of the clock oscillator will be decreased in small steps.
A problem with this type of frequency control is the development of frequency and phase oscillations around the desired frequency and phase. These oscillations may have such an amplitude that the phase control circuit is re-activated and produces phase steps in the locally generated Baud rate clock signal.